1. Field of the Invention
The present invention relates in general to sol materials. More particularly, it relates to sol materials that can be applied to substrates to form microporous dielectric films.
2. Description of the Related Arts
In order to prevent the gel films from shrinking (or even cracking) during drying, it generally requires a long term standing from about 1 day to 1 month for polymer gelation and aging in the sol-gel process for making porous dielectric films. It is believed that the microstructure of the gel is strengthened during the standing process and thus prevents the gel structure from collapsing due to capillary pressure produced by drying. In addition, the shrinking or cracking problem can be solved by surface modification of the resulting films. It is reported that the gel strength can also be improved by surface modification of the preliminary structure of the gel. In any case, however, the standing or the surface modification process has greatly reduced the throughput in practical applications. Accordingly, it would be advantageous to obtain sol materials that on one hand have a long shelf life for storage, and on the other hand can be promptly converted into microporous gels when being used.
U.S. Pat. No. 4,652,467 discloses a method of coating a substrate with a polymeric thin film of predetermined porosity. Prior to depositing the film, the structure of the polymer is controlled by (a) adjusting the water content, the pH, and the temperature of a non-gelled solution, and (b) lowering the temperature of the solution to about 15xc2x0 C. or lower to trap the solution in a state in which, after the deposition step, a coating of the desired porosity will be obtained. Prior to lowering the temperature, the solution is aged at 50xc2x0 C. for a minimum of 6 days to grow polymer to correct size.
U.S. Pat. No. 5,525,643 describes microporous aerogel and xerogel compositions which are prepared by a random polymerization reaction of a silanol-terminated polydimethylsiloxane (PDMS), tetraethylorthosilicate (TEOS), and/or methyltriethoxysilane (MTEOS) to form a gel. The gel is aged for a period of at least about 24 hours before drying to obtain the xerogel or aerogel.
U.S. Pat. No. 5,565,142 describes an extremely porous xerogel dried at vacuum-to-below supercritical pressures but having the properties of aerogels which are typically dried at supercritical pressures. This is done by reacting the internal pore surface of the wet gel with organic substances in order to change the contact angle of the fluid meniscus in the pores during drying. The process consists of a series of aging, washing, and/or surface modification steps which are undertaken upon the wet gel before drying, where the aging step is performed at an elevated temperature for a period of 24-46 hours.
U.S. Pat. No. 5,723,368 describes a porous dielectric material with improved pore surface properties, in which the porous film is treated with a halogen-containing gas to dehydroxylate the pore surface and thereby improve its dielectric properties.
As set forth above, the known materials for forming microporous dielectric films require either aging for at least 24 hours, or surface modification after gel formation to prevent shrinking or cracking of the films. It is therefore an object of the invention to provide a sol material that can be applied to a substrate and directly cured, without aging or the after-surface-modification to prevent shrinkage or cracking, to form a microporous dielectric film.
In addition, to our knowledge the commercially available low-k materials used nowadays, such as FOX-14 (Dow Corning Company) and PAE-2 (Schamacher Company) must be stored at a low temperature of about 5xc2x0 C. It is a further object of the invention to provide a sol material that can be stored at room temperature.
To attain the above objects, the present invention provides a sol material, in the preparation of which an additive is added thereto to control the polymer size and modify the polymer surface as well. The additive, added at the synthesis stage, not only lengthens the shelf life of the sol material, but also enables the coating of which to be directly cured without aging or surface modification. The sol material of the invention is produced by hydrolysis and condensation of the starting materials comprising: 2-60 parts by weight of a silicon alkoxide; 20-98 parts by weight of an alcohol; 0.5-50 parts by weight of an aqueous media; 0.0001-10 parts by weight of a base; 0.001-30 parts by weight of at least one additive; and optionally 0.0001-10 parts by weight of an acid; in which the additive is different from the silicon alkoxide, the alcohol, the base, and the acid.
According to a feature of the invention, the above mentioned additive used for surface modification and particle size controlling is added at the synthesis stage of the sol material. To the contrary, in the prior methods the surface modification is typically carried out when the gel is partially, or completely formed.
In accordance with the invention the sol material thus prepared is stable at room temperature for storage; moreover, it can be promptly converted into a microporous gel with a wide range of porosities without being substantially aged. Therefore, the sol materials according to the invention are particularly suitable for forming dielectric microporous films. The sol materials according to the invention are also applicable to making insulation films, ceramic materials, optical films, and so forth.